This invention relates generally to, synthetic aperture radar systems and, more particularly, to synthetic aperture radar systems used to monitor various phenomena related to the oceans, such as wind speed measurement, ocean wave characteristics, ocean currents, ice flow movement, ship traffic, and so forth. Today, virtually all geological, topographic and mineralogical maps rely on aircraft or satellite photography or radar. Radar has the advantage that it is relatively unimpaired by clouds, rain or fog, but has the disadvantage that, because of longer wavelengths than those of visible light, it needs a very large antenna to achieve the same resolution and image clarity as an optical camera.
More specifically, a conventional radar system obtains range resolution, i.e. distinguishes between target scenery elements at different distances from a radar transmitter/receiver, by transmitting a short radar pulse and measuring the times of arrival of pulses reflected from various elements of the target scenery. In "real aperture" radar systems, the image resolution obtained depends principally on the antenna beam width. Fine azimuth resolution requires a very large antenna, often too large for aircraft or satellite use. Synthetic aperture radar (SAR) systems solve this problem by making use of a variable Doppler shift associated with points in the target scenery. Because of the relative motion between scenery and the aircraft or spacecraft carrying the radar system, images of points in the scenery in front of the antenna are subject to a positive Doppler shift, and points in the scenery to the rear of the antenna are subject to a negative Doppler shift. A SAR system uses a process called azimuth compression or phase history processing, to produce fine resolution images with a relatively small antenna.
SAR systems have been used for some time to monitor the oceans. Radar imagery of the ocean can contain signals due to "hard" targets, such as ships, and more complex signals due to electromagnetic scattering from the water surface itself. When the hard target signals are of primary interest, signal scattering from the water surface is undesirable noise, or "clutter." This clutter can sometimes obscure the hard target signals, and it has long been a goal of designers of radar systems to devise a reliable technique for attenuating or suppressing the clutter due to scattering from the ocean surface.
Another class of applications of ocean radar involves the analysis of movements of the ocean itself, such as in the measurement of ocean currents, wind speed and direction, and so forth. These applications use the signals due to scattering from the ocean surface. In most applications, the information of interest includes the size, strength, and spatial distribution of patches of back-scattered energy, and the motion of these patches. It is well known that radar systems respond primarily to short wavelength ocean waves, and other disturbances from millimeters to centimeters in scale. These short wavelength disturbances are in turn modulated by longer waves, by wind and by other factors. From observations of the properties and motion of the surface patches, various other types of information can be extracted, such as information regarding the sea state, wind speed and direction, ocean currents, the presence of surfactants such as oil from spills, and the presence of anomalous modulation patterns.
Unfortunately, the ever-present motion of the ocean surface causes distortions in the resulting imagery that can be severe. These distortions result from the very power of the SAR, the azimuth compression. The motion of the water surface produces Doppler shifts which "confuse" the SAR processor. Moving portions of the scenery are placed into incorrect azimuth locations, or may be severely smeared in azimuth. Periodic motions of the surface, such as from wave motion, can cause false patterns in the SAR images. Apparent wave motion, wave height, and wave direction can all be dramatically corrupted by a "velocity bunching" phenomenon inherent in SAR systems. This phenomenon is especially severe for radar systems with a large ratio of range to platform speed, referred to as the R/V ratio. Satellite systems, with inherently large range, suffer from this difficulty.
Another difficulty with radar systems is that the correlation of radar reflectivity and Doppler shift results in "radar beats," or long-wavelength patterns in the images. These radar beats contaminate both real aperture and synthetic aperture radar images.
In many applications, a particular type of target lends itself to characterization by a signal pattern that is easily separable from ambient wave pattern, or clutter. For example, separation can be effected in a frequency-wavenumber domain. One unfortunate effect of both radar beats and velocity bunching is that at least some of the clutter is repositioned in the frequency-wavenumber domain such that it interferes more directly with the desired signal and is much more difficult to separate. The net result is a dramatic reduction in detectability of the desired signal.
It will be appreciated from the foregoing that there is still much need for improvement in the field of processing of radar images of the-ocean. In particular, what is needed is an effective technique for eliminating the effects of clutter caused by the ocean surface, and specifically to overcome the problems caused by radar beats and velocity bunching. The present invention is directed to these ends.